US8968204B2 - System and method of perivascular pressure and flow measurement - Google Patents

Abstract

A system and method for measuring fluid flow and pressure in a flexible conduit is disclosed. An embodiment of the system and method uses an ultrasound sensor for determining volume of flow and a tonometric system for determining pressure along a common length of a flexible conduit.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 USC §119 (e) from U.S. provisional application Ser. No. 60/812,845 filed Jun. 12, 2006 with the title of “System and Method of Perivascular Pressure and Flow Measurement”.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A “SEQUENCE LISTING”

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system and method for measuring, a pressure and flow of blood, more particularly it is related to the perivascular measurement of blood flow and pressure at the same location on a blood vessel.

2. Background of the Invention

Blood flow and blood pressure measurement provide useful physiological information in biological systems. If flow and pressure are measured at the same location of a blood vessel, the measurement can allow a determination of the impedance of the tissue or organs to which the vessel is supplying blood.

At present localized pressure measurement in a blood vessel is commonly made with a sensor placed at the end of a catheter tip which is inserted into the blood stream. Because of the invasive nature of the catheter, and the possible change in flow and pressure that can result from introducing a foreign object into the blood stream, use of a catheter has limitations. Also chronic or long term measurements can not be made with a catheter since prolonged insertion of the catheter into the blood vessel causes the patient's immune system to treat the catheter as a foreign body and tissue will form around the catheter thus degrading the ability of the catheter to measure flow and pressure.

Another pressure measurement principle is the tonometric approach, where a pressure sensor is pressed against the outside of a vessel. If certain conditions are met, the pressure sensed in this manner will be equal to the blood pressure inside the vessel. Although the tonometric principle of blood pressure measurement is known and has found use for the non-invasive measurement of intra-arterial pressure (see for instance U.S. Pat. No. 5,284,150) tonometrics has not been adopted as an implantable method for measuring the localized blood pressure of a vessel due to a number of technical problems. A discussion of the general theory behind the technique appears in the article “Arterial Tonometry: Review and Analysis” by Drzewiecki, Melbin and Noordergraaf in the J. Biomechanics Vol. 16 No. 2 pp, 141-152 (1983).

Perivascular measurement of blood volume flow with ultrasound has been a standard technique which has been used since the 1980's. U.S. Pat. No. 4,227,407, describes a perivascular system and method of ultrasound measurement. The principles described in this patent have been applied in the development of transit time flow sensors by Transonic Systems Inc. of Ithaca, N.Y. Doppler flow velocity measurements have been documented since the 1970's, and may be used as an alternate flow measurement approach.

Thus, what is needed is a system and method to obtain in real time pressure and flow readings in a blood vessel or other type of flexible conduit. There is also a need for a system and method to obtain continuous readings of flow and pressure in a blood vessel or other type of flexible conduit over an extended period of time without loss of accuracy in the readings.

SUMMARY

Thus, it is an objective of the present invention to provide a system and method of obtaining at the same location on a blood vessel or other flexible conduit in real time volume flow and pressure measurements. It is a further objective to obtain such readings using a single perivascular sensor without penetration of the vessel or conduit wall. It is yet still a further objective to be able to make these readings in real time over an extended period of time.

The present disclosure achieves these and other objectives by providing: a method for determining fluid flow and pressure of a fluid flowing in a flexible conduit having the steps of: a) making a volume flow or flow velocity measurement using an ultrasound wave beam passed into a conduit at an oblique angle to the a fluid flowing in the conduit; b) flattening a portion of the conduit; and c) obtaining a pressure reading at some or all of the flattened portion of the flexible conduit.

Yet another aspect of the disclosure provides a system for measuring flow volume and pressure in a flexible conduit having: a) a first ultrasound transducer and a second ultrasound transducer detachably positioned adjacent to said location of the flexible conduit, the first transducer being positioned upstream of the second transducer to transmit ultrasound beams between the transducers that illuminate and pass through a full cross sectional area of the conduit; b) a meter operatively connected to the first transducer and the second transducer to control operation of and receive signals from the transducers representative of the characteristics of the ultrasound beam before and after transmission of the ultrasound beam through the conduit to thereby calculate volume flow; c) a pressure transducer detachably positioned on the same location of the conduit against an outside surface of the conduit such that the pressure transducer shapes the adjacent surface of the flexible conduit into a flat surface, and d) operatively connecting to the meter to control operation of and to receive signals from the pressure transducer, which signals are representative of a pressure inside the conduit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by an examination of the following description, together with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a perivascular system for measuring flow and pressure;

FIG. 2 is a full raised view of one embodiment of flow pressure sensor perivascular probe;

FIG. 2A is a side view of a probe head ofFIG. 2 alongline2A with a conduit inserted into the probe head;

FIG. 3 is a front view of another variation of a flow-pressure sensor perivascular probe;

FIG. 4 is a cut away cross sectional view of the probe inFIG. 3 along line IV-IV;

FIG. 5 is a detailed cut away cross sectional view of a portion of the probe ofFIG. 3 along line V-V;

FIG. 6 is a front view of an implantable probe;

FIG. 7 is a cross sectional cut away view of the probe inFIG. 6 along line VII-VII;

FIG. 8 provides a cross sectional review of another variation of an implantable probe;

FIG. 9 is an exploded view of the probe and cuff ofFIG. 8;

FIG. 10 is a view of the top of the cuff ofFIGS. 8 and 9;

FIG. 11 is a schematic diagram of a Doppler ultrasound system; and

FIG. 12 is a crossectional view ofFIG. 11 along line XII.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematic diagram of the major functional components of the present flow andpressure measurement system21.System21 includes aprobe23 that measures both blood flow and pressure at a common location on ablood vessel25 to whichprobe23 has been attached.Probe23 attaches by an electrical lead27 to a combined flow andpressure meter29.Probe23 includesultrasound transducers81A and81B to measure flow and a tonometric pressure measurement sensor which will be described in detail below.

The present system can use a perivascular ultrasound system similar to the one described in U.S. Pat. No. 4,227,407, which is expressly incorporated herein by reference as if set out herein, and discloses the basic features of this type of perivascular ultrasound measurement system.

Meter29 is a standard Transonic HT314 surgical meter made by Transonic Systems Inc. that has the added capability of measuring blood pressure as well as blood flow.Screen31 can display mean volume of flow, flow messages or signal quality information as directed by knob33.Screen35 displays pressure, pressure massages or information on signal quality as directed byknob36. Knob37 controls thegraph printing device41. Knob37 directs the printer to print pressure, flow or a combination of both ongraph printing paper39.

FIG. 2 provides a raised view of one variation of a flow andpressure sensor probe43.Probe43 has ahandle45 that has anelectrical lead47 that passes through the handle and connects with aprobe head49.Probe head49 includes a combination clip andultrasound reflector53 which attaches to ahousing55, which includes bothultrasound transducers81A and81B (not shown inFIG. 2) and a tonometric pressure sensor.Probe43 has aflexible neck59 to allow for the positioning ofprobe head49 such as around a vessel in a patient.FIG. 2A provides a side view ofprobe head49. Theinner surface61 ofclip53 acts as a reflective surface for theultrasound transducers81A and81B located inhousing55. The interior ofhousing55 will be discussed in more detail. As shown inFIG.2A clip53 holds avessel25 securely but detachably againsthousing55.

As noted above, theprobe43 also measures blood pressure of blood flowing in a vessel with a tonometric blood pressure sensing device.FIG. 3 is a close up view of the front of aprobe head73. Ahousing74 contains ultrasonic transducers (not shown inFIG. 3) and atonometric pressure sensor75 that projects out ofhousing74 and abuts against conduit orblood vessel77.Clip79 also projects out ofhousing74 to securely hold conduit orblood vessel77 againsthousing74.Electrical lead80 carries electrical signals between the ultrasonic transducers (not shown inFIG. 3) and thetonometric pressure sensor75 and the flow andpressure meter29.

FIG. 4 is a cut away cross-sectional view ofprobe head73 andvessel77 along line IV-IV inFIG. 3.FIG. 4 shows the position ofultrasound transducers81A and81B that are located insidehousing74.Ultrasound transducers81A and81B are positioned to exchange ultrasound transmissions that are reflected off of aninterior surface87 ofclip79. Readings of flow volume of the blood invessel77 are taken from the ultrasound transmissions produced by the transducers and analyzed as indicated above.Tonometric pressure sensor75 has aflat sensing surface89 that shapes the portion ofvessel77 that the surface abuts against into a flat surface to obtain the necessary readings. Blood flow in the cut away view ofvessel77 is indicated byarrows93.

FIG. 5 is a detailed cut away view ofvessel77,tonometric sensor75 andclip79 along lines V-V ofFIG. 3. InFIG. 5 theflat surface91 shape of thevessel wall96 by theflat sensing surface89 oftonometric sensor75 can be seen. In order to make the pressure measurements with atonometric sensor75, the sensing surface of the tonometric sensor must always conform or shape the adjacent portion of the blood vessel into a flat surface. Tonometric sensing of pressure is based on the principle that when a portion of the surface of a flexible conduit is flattened, the pressure outside and inside the vessel at the flattened portion of the blood vessel will be equal. Thus, a sensor taking a pressure reading at the flattened portion of the surface of the blood vessel will be reading the pressure in the adjacent interior portion of the blood vessel. This concept is based on Laplaces's law for a pressure gradient across a vessel's wall which is expressed in the following equation:

$\begin{array}{cc}\mathrm{Pout}-\mathrm{Pin}=\frac{T}{r}& \left[1\right]\end{array}$
In this equation Pout is the pressure outside the wall of the vessel and Pin is the pressure on the inside of vessel. T is the vessel wall tension and r is the radius of the vessel. Equation1 can be modified as follows by simple algebraic manipulation:

$\begin{array}{cc}P\mathrm{out}=\frac{T}{r}+Pin& \left[2\right]\end{array}$
If the wall of the vessel is then flattened in effect then the radius r goes to infinity r=∞. Thus substituting this value for r in the above equation results in T/r going to zero so the above equation can be reduced to the following:
Pout=Pin  [3]
Thus as can be seen the pressure differential across the vessel wall at the flattened portion goes to zero ΔP→0.

Thetonometric sensing surface89 is flat to thereby conform or dispose the adjacent vessel wall into a flat and rigid surface necessary for the pressure measurement. Various types of semiconductor sensing elements could be embedded in theflat surface89 to make the pressure measurements at the flattenedsurface91. These could be capacitive type of pressure sensors, strain gauges, etc. These devices are typically made of piezoelectrical active types of materials that are naturally sensitive to the application of mechanical stress. As can be seen inFIG. 4,electrical connections97 run from theultrasonic transducers81A and81B as well as tonometric sensor85 up throughelectrical line conduit80 to the flow-pressure meter29 (not shown).

FIG. 6 provides an enlarged view of another variation of a flow-pressure sensor probe101. The variation of the disclosure inFIG. 6 would be implanted into a test subject such as a laboratory rat, sheep, horse etc. for chronic, long term measurements. Theprobe101 would naturally be placed around ablood vessel103 by inserting the blood vessel through agap105 formed byhousing107 andclip109. Sincevessel103 is flexible and easily deformable thevessel103 may be inserted throughgap105.Probe101 is sized such that asensing surface113 of atonometric sensor111 abuts firmly up against an outside wall ofvessel103 and forms the flat surface described previously that allows for the direct tonometric measurement of pressure. Alternatively, an insert sized to fit into theprobe101 could be use to hold the vessel, this will be discussed below.Electrical conduit115 passes out through the skin of the test animal and directly attaches to a flow-pressure meter29 (not shown) by a long lead or alternatively attaches to a telemetric pack attached to the outside of the animal and the readings are conveyed by wireless transmission to the flow-pressure sensor meter29 or computer (not shown). Alternately,electrical conduit115, may connect to a fully implanted signal telemetry device (not shown) in the subject.

FIG. 7 is a cross sectional cut away view along line VII-VII ofFIG. 6. InFIG. 7 a flattenedportion117 ofvessel103 wall can be seen. When a probe, such asprobe101 is chronically implanted,overtime tissue119 may grow aroundprobe101 and betweenprobe housing107 andvessel103. However,tissue119 does not negatively affecttonometric sensor111 because at flattenedsurface117 thetissue119 atrophies and relies on sensingsurface113 for support. This reliance bytissue119 on thesensing surface113 enhances the operation of thetonometric sensor111, as the interposedfibrous tissue119 becomes passive and thus incapable of altering pressure invessel103. Additionally, thetissue growth119 betweenhousing107 andvessel103 forms a uniform transition betweenultrasound transceivers81A and81B located in housing (not shown inFIG. 7) andvessel103, which will reduce motion artifacts.

FIG. 8 is another variation of chronically implantable type of probe. In this variation numbering of the various parts disclosed inFIGS. 6 and 7 has been retained. The added feature is an insert orcuff121.Cuff121 is made of an acoustically compatible, flexible and reliant material.Cuff121 is sized to fit intohousing107 ofprobe123. As depicted inFIG. 8cuff121 has anopening125 sized conformal tovessel103, and is designed to fit securely but detachably inhousing107 ofprobe123.Cuff121 is made of a material that is acoustically compatible and biocompatible withvessel103. Being acoustically compatible with the vessel and blood, the material will not deform the ultrasound fields that derive flow readings fromvessel103. This increases the accuracy of theprobe123. Biological compatibility reduces rejection of thecuff121 by the body. A material that meets this criteria is Pebax® (Elf-Autchem). A detailed discussion of the insert orcuff121 appears in Copending provisional application Ser. No. 60/881,826 filed Jan. 23, 2007 and titled “Disposable Insert for a Perivascular Probe Head,” which is incorporated herein by reference.

FIG. 9 provides an exploded view ofcuff121 andhousing107 into whichcuff121 is inserted in a secure but detachable fashion.FIG. 10 provides a top view ofcuff121 along line X-X ofFIG. 9. As can be seencuff121 has ahole133 in a top of the cuff to receivesensor111.

Since volume flow and pressure can be measured on the same location of a blood vessel, these measurements make it possible to calculate the impedance of the tissue or organ(s) supplied by the vessel being measured. Impedance Z can be calculated by dividing pressure by flow, the equation would be as follows where P is pressure and Q is flow:

$\begin{array}{cc}Z=\frac{P}{Q}& \left[4\right]\end{array}$
Values for impedance can be determined with either flow volume, as is the case with the use of transit time ultrasound or with flow velocity as is the case with back scattered Doppler ultrasound system that are discussed below.

One preferred embodiment of the disclosure employs a transit time ultrasound sensor, which fully illuminates the cross sectional area of thevessel103 with bidirectional beams of ultrasound. It is within the spirit of the disclosure to employ other sensors for the measurement of flow.

In another variation, Doppler ultrasound sensors could be used in place of transit time flow sensors.FIG. 11 provides a schematic diagram of a Doppler ultrasound system with the combinedtonometric sensor111 andDoppler sensor149adjacent vessel103. In Doppler ultrasound systems,ultrasound151 is directed into thevessel103 at an oblique angle. For a detailed discussion of how a Doppler ultrasound sensor functions, publications and textbooks known by those of ordinary skill in the art adequately sets forth the level of those skilled in the art. In one variation the Doppler ultrasound sensor could be limited to reading flow velocity and not volume flow. However, by taking a series of readings over a cross sectional area of thevessel103, the internal diameter of thevessel103 may be determined as well and volume flow may be measured.FIG. 12 provides a cross sectional view of the system ofFIG. 11 along line XII, whereDoppler ultrasound sensor149 takes readings of the flow speed at several different cross sectional points160A,1608,160C,160D and160E ofvessel103 in order to thereby estimate volume flow.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made to it without departing from the spirit and scope of the invention.

Claims (26)

The invention claimed is:

1. A method for determining fluid volume flow and a pressure of a fluid flowing along a flattened section of a flexible conduit, the method comprising the steps of:

making at least two passes of an ultrasound beam emanating from a housing through the flexible conduit at an oblique angle to the fluid flowing in the flexible conduit, wherein the two passes are in opposite directions along a same path, and wherein the ultrasound beam illuminates the fluid flowing in the flexible conduit on each pass;

determining a change in a characteristic of the ultrasound beam after each pass through the flexible conduit;

determining the fluid volume flow;

flattening with a pressure sensor connected to the housing, at a position where the ultrasound wave beam passes through the fluid, a portion of an outside surface of the flexible conduit to form a stationary flat surface without creating a full occlusion in the flexible conduit and;

obtaining a pressure reading from the outside surface of the flattened portion of the flexible conduit.

2. The method ofclaim 1 comprising the further step of: calculating an impedance to flow of a portion of the flattened conduit corresponding to the obtained pressure reading and the determined fluid flow volume.

3. The method ofclaim 1 wherein the conduit is a blood vessel and the fluid is blood.

4. A device for measuring a volume flow and a pressure within a stationary flattened length of a flexible conduit, the device comprising:

a first ultrasound transducer and a second ultrasound transducer configured to be removably located adjacent to the stationary flattened length of the flexible conduit, the first transducer being positioned upstream of (i) the stationary flattened length and (ii) the second transducer, such that transmission of ultrasound beams between the transducers illuminate and pass through a cross sectional area of the stationary flattened flexible conduit;

a pressure sensor configured to be removably located adjacent to an outside surface of the flexible conduit along the length of the conduit such that the pressure sensor is configured to shape a surface of the flexible conduit into the stationary flattened surface; and

a meter operatively connected to the first transducer and the second transducer, and configured to determine the volume flow; and the meter further being operatively connected to the pressure sensor to determine the pressure inside the flexible conduit.

5. The device ofclaim 4 wherein the flexible conduit is a blood vessel.

6. The device ofclaim 4 wherein the flexible conduit is a deformable tubing.

7. The device ofclaim 4 wherein the first transducer and the second transducer are positioned on opposite sides of the flexible conduit.

8. The device ofclaim 4 wherein the first transducer and the second transducer are positioned on a same side of the flexible conduit and wherein their beams are directed to an ultrasonic reflective surface on an opposite side of the flexible conduit.

9. The device ofclaim 4 wherein the pressure sensor is one of a strain gauge and a capacitive sensing device.

10. The device ofclaim 4 wherein the first transducer and the second transducer are configured to transmit ultrasound beams simultaneously.

11. A probe for determining volume flow and pressure of a fluid in a flexible conduit, the probe comprising:

a.) a frame configured to removably receive a length of the flexible conduit;

b.) a first transducer mounted to the frame, and configured to transmit a first ultrasound beam along an oblique angle path across the flexible conduit so that the first ultrasound beam illuminates fluid present in the flexible conduit;

c.) a second transducer mounted to the frame, and configured to receive the first ultrasound beam, and further configured to transmit a second ultrasound beam to the first transducer across the flexible conduit along the oblique angle path;

d.) a flat tonometric sensing surface having a pressure sensor mounted to the frame, the flat tonometric sensing surface configured to flatten the wall portion of the flexible conduit received within the frame; and

e.) a meter operably connected to the first transducer, the second transducer and the sensor, and configured to determine the volume flow and the pressure of the fluid in the flexible conduit.

12. A method for determining a flow property and a pressure of a fluid flow passing within a flexible conduit, the method comprising the steps of:

a) generating from a position adjacent to and outside the flexible conduit at least one ultrasound wave that passes through a cross sectional area of the flexible conduit at an oblique angle to fluid flowing within the flexible conduit;

b) receiving at least one ultrasound wave after passage through a cross sectional area of the flexible conduit;

c) determining flow from the received ultrasound wave;

d) flattening to a fixed position an outside portion of the flexible conduit along a length of the flexible conduit through which at least one ultrasound wave passes by exerting a flat surface of a pressure sensor to the outside portion of the flexible conduit; and

e) taking a pressure reading from the pressure sensor.

13. The method ofclaim 12 comprising the further step of determining one additional physical parameter from at least one of the pressure reading and the received at least one ultrasound wave.

14. The method ofclaim 13 wherein the further step of determining one additional physical parameter is determining an organ impedance in the vicinity of the flexible conduit.

15. The method ofclaim 13 wherein the further step of determining one additional physical parameter is determining a tissue impedance in the vicinity of the flexible conduit.

16. The method ofclaim 12 wherein the flexible conduit is a blood vessel.

17. The method ofclaim 12 wherein the step of generating at least one ultrasound wave comprises generating the at least one ultrasound wave with a transit time ultrasound sensor to illuminate a full cross section of the flexible conduit.

18. The method ofclaim 12 wherein the step of generating at least one ultrasound wave comprises generating the at least one ultrasound wave with a Doppler ultrasound transducer.

19. A probe assembly for monitoring flow in a conformable conduit, the probe assembly comprising:

(a) a housing sized to receive a length of the conformable conduit;

(b) a pressure sensor connected to the housing and having a substantially flat outer surface configuration at a fixed location such that a portion of the length of the conformable conduit locally conforms to the flat outer surface; and

(c) a first transducer connected to the housing and configured to illuminate a cross section of the conformable conduit with a signal along a signal path, at least a portion of the signal path being within the locally conformed portion of the length of the conformable conduit.

20. The probe assembly ofclaim 19, wherein the pressure sensor directly contacts an outside surface of the conformable conduit.

21. The probe assembly ofclaim 19, wherein the housing includes a reflector in the signal path spaced from the first transducer.

22. The probe assembly ofclaim 19, further comprising a second transducer spaced from the first transducer, wherein the housing includes a reflector in the signal path intermediate to the first transducer and the second transducer.

23. A method of monitoring a flow in a conformable conduit, the method comprising:

(a) passing an ultrasound signal from a transducer in a probe housing through a length of the conformable conduit; and

(b) sensing a pressure of a locally conformed region of the conformable conduit along the length of the conformable conduit, the locally conformed region having an outside wall of the conformable conduit being substantially flat and stationary.

24. The method ofclaim 23, wherein the transducer is an upstream transducer and passing the ultrasound signal includes passing the ultrasound signal through the locally conformed region of the conformable conduit from the upstream transducer to a downstream transducer.

25. The method ofclaim 23, wherein the transducer is an upstream transducer and further comprising retaining the upstream transducer and a downstream transducer in the probe housing, the ultrasound signal passing from the upstream transducer to the downstream transducer through the conformable conduit.

26. The method ofclaim 23, wherein the transducer is an upstream transducer and further comprising retaining the upstream transducer, a downstream transducer and a pressure sensor in a probe housing, the ultrasound signal passing from the upstream transducer to the downstream transducer through the conformable conduit, and the pressure sensor intermediate the upstream transducer and the downstream transducer.

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